The present invention relates to hydraulic pressure control valves which are mounted respectively to discharge sections of respective brake master cylinders for automotive vehicles and, more particularly, to a hydraulic pressure control valve in which a control action of the control valve is released by utilization of excessive movement of a piston within the master cylinder when malfunction occurs in one of dual brake systems.
A hydraulic pressure control valve of the kind referred to above is known from, for example, Japanese Patent Provisional Publication No. SHO 60-226347 (corres. to European Patent Application No. 0 157 158A2). The known hydraulic pressure control valve comprises a valve body whose one end is mounted to a discharge section of a master cylinder. A valve element is arranged within the valve body and has a stud which projects outwardly from the one end of the valve body. The valve element is seated against a valve seat formed in an end of a stepped piston which is arranged within the valve body. An O-ring seal element is arranged within an annular recess formed in the valve body at a location of the end of the stepped bore, for sealing between the valve body and an outer peripheral surface of the stepped piston.
Since the seal element is merely received in the annular recess, the seal element tends to be disengaged from or to come out of the annular recess by reciprocal movement of the stepped piston, at operations such as vacuum bleeding and the like. If the seal element comes out of the annular recess, the sealing between the valve body and the outer periphery of the stepped piston is lost.
In connection with the above, the aforesaid EP 0 157 158A2 discloses an annular axial projection which is provided on the valve element at a location of an end thereof, which faces toward the valve seat. The annular axial projection extends axially of the valve element toward the seal element. The annular axial projection is effective in prevention of the seal element from coming out of the annular recess. Prior to incorporation of the hydraulic pressure control valve into the master cylinder, however, the valve element tends to laterally fall down considerably. Thus, it cannot be expected for the annular axial projection to prevent the seal element from coming out of the annular recess.
Further, as disclosed in Japanese Utility Model Provisional Publication No. SHO 63-61373, an annular lateral projection is provided on the valve body at a location of an end of the annular recess, which opens to a valve chamber defined at the one end of the valve body. The annular lateral projection extends radially inwardly from the valve body toward the end of the stepped piston at which the valve seat is formed. The annular axial projection is effective in prevention of the seal element from coming out of the annular recess. However, the annular lateral projection makes it difficult to insert the seal element into the annular recess. In order to facilitate incorporation of the valve element into the annular recess, the annular end of the annular recess, which opens to the valve chamber, must have a sufficient space by means of, for example, taper means.
Furthermore, since the stud of the valve element considerably projects or protrudes outwardly from the one end of the valve body, there is the following problem, prior to the control valve being incorporated in the master cylinder. That is, when the hydraulic pressure control valve is handled for inspection of a control characteristic of the hydraulic pressure control valve, for packing the same, or the like, there may be such a fear that the projecting stud on the valve element is abutted against another member so that the valve element is tilted or falls down. If the tilting or falling-down of the valve member is excessive, the valve element moves beyond a step at an outer peripheral edge of the valve seat so that the valve element is fitted into the annular recess located radially outwardly of the step. Thus, there may be a case where the valve element cannot be returned to its regular or right posture. If it is not known that the valve element is fitted into the annular recess and if the hydraulic pressure control valve is incorporated in the master cylinder as it is, there is such a fear that the valve element is damaged, particularly, in case where the valve element is made of plastic material, the valve element is broken.
In view of the above problem, the applicant of this invention has proposed, in Japanese Utility Model Application No. SHO 63-112031, such a technique that a cap member for restricting movement of the valve element is mounted to the one end of the valve body thereby preventing the valve element from falling down excessively and thereby previously avoiding damage on the valve element.
However, subsequent consideration indicates the fact that, in case where the cap member is made of plastic material which is easy in manufacturing, the cap member per se is moved easily by an external force. Thus, there may be a case where it is impossible to surely prevent the valve element from falling down excessively.